It is believed they are being held somewhere in the vast forested areas that stretch from near Chibok into neighbouring Cameroon.

More protests have been held in the British capital, London, and Nigeria's main city, Lagos, on Friday.

A Foreign Office statement said the British experts would be working closely with their US counterparts.

"The team will be considering not just the recent incidents but also longer-term counter-terrorism solutions to prevent such attacks in the future and defeat Boko Haram," it said.

Earlier, US Secretary of State John Kerry said: "Our inter-agency team is hitting the ground in Nigeria now and they are going to be working in concert with President Goodluck Jonathan's government to do everything that we possibly can to return these girls to their families and their communities."

"We are also going to do everything possible to counter the menace of Boko Haram," he said.

On Wednesday, President Barack Obama said the team comprised personnel from military, law enforcement and other agencies.

He said he hoped the kidnapping would galvanise the international community to take action against Boko Haram.

President Goodluck Jonathan has accepted offers of help from the international community

President Jonathan, speaking at the World Economic Forum being hosted in the capital, Abuja, said the abduction of the girls could be a turning point in the battle against Boko Haram.

"I believe that the kidnap of these girls will be the beginning of the end of terror in Nigeria," he said.

Boko Haram at a glance

The government has vowed to defeat Boko Haram

Founded in 2002

Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" in the local Hausa language